The present invention relates to antennas intended to be affixed to surfaces, for use with radio frequency devices such a cellular phones, GPS location systems, and other RF applications. The present invention further relates to a method for manufacturing conductive patterns on substrates.
Many applications currently exist where an RF antenna is provided in order to enable communicationxe2x80x94for example, cellular telephones, GPS systems, wireless data networks and the like. In some cases the antenna is provided with the device, for example as a stub unit on a cellular phone. In other cases, however, it is necessary to provide an externally connected antenna. Further, in applications such as in-car use of cellular phones, it is desirable to provide an additional antenna to boost signal strength. Traditional antennas for this purpose have been generally externally mounted on the vehicle. This increases wind noise, is prone to vandalism, and detracts from the appearance of the vehicle.
For any antenna application of this type, various issues need to be considered. Apart from addressing the problems mentioned above, the antenna should provide maximum capture area, whilst ideally being visually unobtrusive. It should be simple to install, yet electrically and structurally reliable.
It has been proposed to provide an antenna by adhering an array to the inside of a window of a motor vehicle. U.S. Pat. No. 5,363,114 to Shoemaker describes a planar, serpentine antenna which is adhered to a carrier layer, and which is then adhered to a suitable vehicle surface. The antenna is disclosed as having a serpentine patterned arrangement.
It is an object of the present invention to provide an improved antenna for mounting on planar surfaces.
According to one aspect the present invention provides a planar antenna comprising a rectangular conductive element formed from two square elements, the square elements being defined within the rectangle by a centrally located and bifurcated return conductor having two legs, each square element being connected at one end to a connector element and at the other end to said return conductor, wherein the dimensions of the square elements are chosen so as to maximize gain for selected radio frequencies, characterised in that the antenna further includes one or more additional but partial square elements, each additional partial square element terminating adjacent the respective square element at one end and being defined by one leg of the return conductor on one side.
It will be understood that the term planar is intended to mean both flat surfaces and smooth curved surfaces, such as for example the shape of a vehicle windshield.
The inventive antenna arrangement has a number of advantages over the existing designs. The intended applications, where the antenna is adhered to an existing surface such as a window, do not require that the conductive elements be structurally rigid themselves, thereby enabling the use of a sparse geometry. This also enables the antenna to have a relatively large capture area, as it is mounted on a surface and not freestanding. Further, as there are elements disposed both horizontally and vertically, the antenna can receive either vertically or horizontally polarised signals well, which is advantageous in applications where scattering due to buildings and other structures occurs.
The present invention also provides a method for providing conductive elements on a substrate, including the steps of:
printing a desired conductor pattern onto a substrate, using conductive ink; and
electrodepositing further conductive material onto the conductor pattern, using the pattern formed from conductive ink as an electrode in an electroplating process.
The conductive material may be conveniently copper. The parameters of the electroplating process will depend upon the process selected, but should be such as to provide an adequate thickness of copper, but not so much that too much copper is deposited and the pattern becomes vulnerable to mechanical failure. The inventor has found that in the cellular phone application a thickness of about 25 microns is suitable.
The pattern is suitably printed using a screen printing process. In practice, a large sheet of flexible material can be printed and cut using a suitable tool to provide many antenna arrays.
After depositing, preferably a double sided adhesive film, preferably transparent, is applied both to provide a mechanism for adhesion to the desired surface, and to inhibit corrosion of the copper.
The inventors have investigated various methods for practical manufacture of the antenna. Whilst the invention arose in this context, it will be understood that the inventive method can equally be applied to manufacture of other conductor on substrate devices. The use of conductive ink alone did not provide suitable resistive properties for the antenna, and the addition of electrodeposition to the printing approach was only arrived at after significant trial and error.